1. Field of the Invention
The present invention relates generally to an apparatus and method for providing Digital Multimedia Broadcasting (DMB) service, and in particular, to a gap filler apparatus and method for providing DMB service using cyclic delay diversity.
2. Description of the Related Art
DMB service is the next generation digital broadcasting service in which subscribers can enjoy high-quality, multi-channel multimedia broadcasting. Generally, DMB service is provided to subscribers through a portable terminal or a vehicle-mounted terminal (hereinafter simply referred to as a “terminal”), and can also be provided through a computer equipped with a DMB receiver. DMB service can transmit CD-level high-quality multimedia streams via a wireless network even at a moving velocity of 200 Km/h, and can be classified into satellite DMB service and terrestrial DMB service according to transmission medium used.
Herein, satellite DMB service refers to service for providing digital broadcasting using a satellite and gap fillers installed in the ground, and terrestrial DMB service refers to service for providing digital broadcasting through the gap fillers. Terrestrial DMB service is technology provided by adding Motion Picture Experts Group-4 (MPEG-4) source coding also in order to transmit a moving image based on European Research Coordination Agency project-147 (Eureka 147), which is a standard for a European Digital Audio Broadcasting (DAB) system, and it is expected that terrestrial DMB service will be popularized in the near future.
Unlike the general cellular network, a broadcasting network is preferably featured by a single-frequency network (SFN). Therefore, in the broadcasting network, one terminal receives signals in the same frequency band transmitted from a plurality of gap fillers at regular intervals. Because terrestrial DMB service uses Orthogonal Frequency Division Multiplexing (OFDM) as a transmission scheme, if time intervals between the gap fillers fall within a guard interval of an OFDM symbol, they can be regarded as multiple paths of a general wireless fading channel. However, if a time interval between multipath signals generated by a delay exceeds a length of the guard interval, it may undesirably lead to inter-symbol interference (ISI) and inter-carrier interference (ICI).
FIG. 1 is a diagram illustrating a configuration of a conventional SFN. In FIG. 1, a plurality of gap fillers 102, 104, and 106 amplify the intact broadcasting signals transmitted from a main station 100 and retransmit the amplified broadcasting signals to a terminal 108. That is, the gap fillers 102, 104, and 106 simply serve to compensate for a power loss due to a path loss.
FIG. 2 is a diagram illustrating an internal structure of a conventional gap filler. The conventional gap filler simply performs analog filtering through a reception band-pass filter (Rx BPF) 200 and a transmission band-pass filter (Tx BPF) 204 without demodulation and decoding operations, and simply performs signal amplification using an amplifier 202. In such a gap filler, because there is almost no time delay caused by signal processing, such as demodulation or decoding, signals transmitted from multiple gap fillers to a terminal have time differences corresponding to the distances between the terminal and the gap fillers.
Generally, a Coded-OFDM (COFDM) system shows better performance as frequency selectivity of a channel is higher, i.e., as a coherence bandwidth is narrower, because when the frequency selectivity is low, a reception signal may suffer from considerable attenuation due to fading at a certain time. However, when the frequency selectivity is high, reception signals are uniform to some extent in terms of the total power, and although there are many nulls at a particular frequency, they can be compensated for by forward error correction, thereby improving performance.
In order for a terminal to increase the frequency selectivity for reception performance improvement, the number of multiple paths of a fading channel should increase. Therefore, it is preferable to provide as many paths as possible, as long as they fall within the guard interval. In particular, when a terminal is located in a position where it secures a line of sight (LOS) because it is adjacent to a transmitter or has a less number of obstacles between the terminal and the transmitter, the terminal may have a fewer signal paths. However, a terminal located in such a position is susceptible to fading. Therefore, in order to guarantee reception performance on an appropriate level, the terminal must transmit signals with unnecessarily high power.
In order to address these problems, a cellular OFDM system has introduced a method in which each base station creates effective channels with multipath channels using a cyclic delay for transmission signals. In multiple transmission antennas of a base station a codeword sequence (X0,X1, . . . ,XN−1) with a block length N is modulated with N subcarriers. Signals OFDM-modulated by Inverse Discrete Fourier Transform (IDFT) are given by Equation (1):
                                          x            n                    =                                    1                              N                                      ⁢                                          ∑                                  k                  =                  0                                                  N                  -                  1                                            ⁢                                                X                  k                                ⁢                                  ⅇ                                      j2π                    ⁢                                                                                  ⁢                    k                    ⁢                                                                                  ⁢                                          n                      /                      N                                                                                                          ,                  n          =          0                ,        1        ,        …        ⁢                                  ,                  N          -          1                                    (        1        )            
The signals are inserted into a time-domain sequence {xn} on the assumption that a length G of a cyclic guard interval is equal to the number M of antennas (G=M). Then, the results are shown in Equation (2):{tilde over (x)}(n+G)N+G=x(n)N, n=0,1, . . . , N+G−1  (2)
where (n)N denotes a residual obtained by dividing n by N.
FIG. 3 is a block diagram illustrating a transmitter for performing basic cyclic delay diversity modulation using multiple carriers in a conventional DMB system. In the transmitter of FIG. 3, a transmission signal is input to an encoder 300. The encoder 300 encodes the transmission signal and outputs the coded transmission signal to a serial-to-parallel (S/P) converter 302. The S/P converter 302 converts the coded transmission signal into parallel signals, and outputs the parallel signals to an Inverse Fast Fourier Transform (IFFT) unit 304. The IFFT unit 304 modulates the parallel signals with N subcarriers, and outputs the IFFT-modulated signals to a parallel-to-serial (P/S) converter 306. The P/S converter 306 converts the IFFT-modulated signals into a serial signal. A codeword sequence (X0,X1, . . . ,XN−1) with a block length N of the serial signal is delayed by each of delays 308, 309, and 310 by a delay interval T, and transmitted to a wireless network via M transmission antennas. The signals transmitted to the wireless network via M transmission antennas have a tapped delay line structure with a length M−1.
The delay interval is equal to a symbol interval T of the time-domain sequence {xn}. In this case, the codeword C is given by Equation (3).
                    C        =                  (                                                                      x                  0                                                                              x                  1                                                            …                                                              x                                      N                    -                    1                                                                                                                        x                                      N                    -                    1                                                                                                x                  0                                                            …                                                              x                                      N                    -                    2                                                                                                      ⋮                                            ⋮                                            ⋰                                            ⋮                                                                                      x                                      N                    -                    M                    +                    1                                                                                                x                                      N                    -                    M                    +                    2                                                                              …                                                              x                                      N                    -                    M                                                                                )                                    (        3        )            
Referring to Equation (3), in FIG. 3, an Mth transmission antenna #(M−1) transmits a sequence (xN−m,xN−m+1, . . . ,xN−m−1) obtained by cyclic-shifting a sequence (x0,x1, . . . ,xN−1) of a first transmission antenna #0 by a symbol interval T, M times. The codeword C defined in Equation (3) is called a cyclic delay codeword.
A base station or a mobile terminal using multiple antennas can obtain additional frequency diversity gain by cyclic-delaying OFDM signals for the individual antennas by a predetermined interval before transmission, using the cyclic delay diversity modulation scheme.
Assuming that an OFDM signal xn is transmitted via M antennas, a time interval of the OFDM signal xn is denoted by T, a maximum time delay in a frequency selective fading channel is denoted by τmaxT, energy of the OFDM signal is denoted by Es and a channel response is denoted by hn, a received signal yn is defined as shown in Equation (4).
                              y          n                =                                                            E                s                                      ⁢                          x              n                        *                          h              n                                =                                                                      E                  s                                            ⁢                                                ∑                                      l                    =                                          -                      ∞                                                        ∞                                ⁢                                                      h                    l                                    ⁢                                      x                                          n                      -                      l                                                                                            =                                                            E                  s                                            ⁢                                                ∑                                      l                    =                    0                                                                              τ                      max                                        -                    1                                                  ⁢                                                      h                    l                                    ⁢                                      x                                          n                      -                      l                                                                                                                              (        4        )            
In the cyclic delay diversity modulation scheme, a guard interval of the OFDM signal is set to τG1T. Preferably, the guard interval should be set longer than the maximum time delay τmaxT of the OFDM signal. The guard interval τG1T of the OFDM signal, which is set longer than the maximum time delay τmaxT, can be expressed as in Equation (5).τG1T≧max(MT,τmaxT)  (5)
Assuming that antennas of the transmitter are individually independent of antennas of a receiver and in Equation (4), a channel impulse response between an lth transmission antenna and a reception antenna is denoted by hnl, there is no channel impulse response after the maximum time delay of the OFDM signal.
If a frequency response for a channel received at a reception antenna from an lth transmission antenna is denoted by Hkl, an effective channel in a frequency domain at the receiver can be represented by Equation (6).
                              H          k                =                              ∑                          l              =              0                                      M              -              1                                ⁢                                    H              k              l                        ⁢                          ⅇ                                                -                  j2π                                ⁢                                                                  ⁢                k                ⁢                                                                  ⁢                                  l                  /                  N                                                                                        (        6        )            
That is, the cyclic delay diversity modulation scheme of FIG. 3 converts an effective channel from a flat fading channel to a multipath fading channel from the standpoint of the receiver.
Fading gain obtainable by delaying an OFDM signal before transmitting it via different antennas can be equivalent to fading gain obtainable from different paths in a multipath channel. Herein, the cyclic delay diversity modulation scheme will be extended not only to a multiantenna environment but also to a cellular environment. That is, it is assumed that different cyclic delays are given even for the transmission signals transmitted from individual base stations.
FIG. 4 is a block diagram illustrating a system for applying a cyclic delay diversity modulation scheme for transmission signals transmitted from individual base stations. Referring to FIG. 4, each of base stations BS1, BS2, and BS3 includes a plurality of transmission antennas. The base stations BS1, BS2, and BS3 simultaneously transmit the same data stream 400. Each base station can obtain macro diversity by applying an appropriate cyclic delay through a cyclic delay diversity unit 430. Each base station can obtain antenna diversity either for its own antennas or for the total antennas. The latter case may cause an increase in complexity of a receiver.
FIG. 5 is a diagram illustrating a combination of antenna diversity and cyclic delay diversity in a conventional cellular environment. As illustrated in FIG. 5, a terminal 108 can obtain antenna diversity either from base stations 102, 104, and 106, or from the total antennas.
The broadcasting service is most disadvantageous in that a reception capability of the terminal is low in the blanket area. If a gap filler applies cyclic delay diversity using the characteristics of an OFDM signal, it is possible to obtain additional frequency diversity gain, contributing to possible performance improvement. That is, each base station in the cellular environment illustrated in FIG. 5, under the control of its upper layer of a base station controller, can adjust a cyclic delay value of an OFDM transmission signal and can also process digital signals.
However, the terrestrial DMB system cannot apply the cyclic delay diversity taking the structure of the conventional gap filler into consideration because a gap filler for terrestrial DMB service simply amplifies signals without a separate demodulation means and retransmit the intact analog signals. Accordingly, the gap filler for terrestrial DMB service cannot apply the cyclic delay diversity.
Although there is a possible scheme in which the gap filler applies the cyclic delay diversity by demodulating a received OFDM signal, this scheme causes an excessive increase in the manufacturing cost of the gap filler. In addition, if a delay in the gap filler due to the demodulation operation exceeds the guard interval of the OFDM symbol, it serves as an interference signal, making it difficult to make the best use of the SFN.